STRATIGRAPHY Teacher Guide including Lesson Plans, Student Readers, and More Information Lesson 1 - What is stratigraphy? Lesson 2 - Correlation Activity Lesson 3 -Geologic Time Lesson 4 - Earth’s History - Lab Lesson 5 - Environments through Time designed to be used as an Electronic Textbook in class or at home materials can be obtained from the Math/Science Nucleus Math/Science Nucleus © 2001 1 EARTH SCIENCES - STRATIGRAPHY Lesson 1 - What is stratigraphy? MATERIALS: Objective: Students learn the how reader stratigraphy developed. Teacher note Stratigraphy is usually overlooked in beginning Earth Science classes. However, it is an important tool to the petroleum and mining industries. Stratigraphy is the analysis of different rock formation through time and changing environments. Usually it is associated with sedimentary rocks because they follow predictable rules as they are deposited. Igneous and metamorphic rocks are more random on how they are deposited. Rocks repeat themselves through geologic time and are difficult to determine the age when they were deposited. A granitic sandstone that was formed millions of years ago, can resemble a granitic sandstone of today. However, if you look at the fossils within these sedimentary rocks, they can provide clues on timing of deposition. For example, rocks with trilobites in them will not be forming today because there are no trilobites. The evolution of organisms helps us to compare the different rock strata and determine which one is older or younger. Fossils give us clues for which strata is younger or older. It was the layers of rocks that made early geologist think about what could have created these layers. After careful analysis, many geologists became convinced that it took a lot of time to create these layers. Serious debates over the age of the Earth were started by geologists stating that the Earth was older than one thousand years. Stratigraphy is built on the concept that the present is the key to understanding the past. The same processes that create the rocks today were in operation in the past. This helps us reconstruct how the strata was deposited in the past. The following website has more information on stratigraphy and the scientists involved in changing how we think: http://turnpike.net/~mscott/index.htm Math/Science Nucleus © 2001 2 Walking along cliffs that were cut by ocean waves or rivers have always made humans wonder how they were formed. The time required to create such majestic towers has created serious debate, both scientific and religious. Geologic time was very difficult for scientists to "discover.” It was not until the mid 18th century that James Hutton, a Scottish geologist, realized that the Earth was many millions of years old. This was an unimaginable idea because people in his day believed the Earth was only a few thousand years old. Hutton tried to James Hutton wondered about the rocks from the Dorset, develop scientific United Kingdom coastline methods to determine the time required for every day geologic processes and compare with the past. For example he tried to calculate mud accumulating in the ocean today, to figure out how much time had passed since the formation of the Earth. He used the term “uniformitarism” to compare James Hutton the present day rock cycle with the past rock cycle. From these comparisons you can interpret how rock layers or strata were formed but not the length of time. You can determine which stratum is younger or older, just by the position of the strata. Since most rocks on the surface of the Earth are sedimentary, early geologists used them to look for answers to the age of the Earth. The birth of stratigraphy has its roots in scientists trying to determine the age of the Earth. They made simple predictions by looking at Deposition of fossils and sediment sedimentary processes going on today. Sedimentary layers with fossils Geologists started to realize that you can trace certain strata by comparing the fossils that it contains. The use of fossils became an important tool to unravel the history of the Earth. Math/Science Nucleus © 2001 3 Nicholas Steno, a Danish physician living in Italy in 1669 proposed that the Earth’s strata accumulated with three basic principles. Steno pointed out obvious, but overlooked principles of sediment accumulation. They included the Principle of Original Horizontality, Principle of Superposition, and Principle of Original Continuity. If sediments accumulate in a large basin, the laws of gravity will deposit the beds, horizontal to the surface of the Earth. Beds can “pinch out” along the sides of the basin as in the figure below. The Principle of Superposition states that in a sequence of sedimentary rock layers, the bottom layers are older than the top layers. The bottom layers were deposited first. In the figure below A is the oldest bed and G is the youngest. The Principal of Original Continuity states that the beds can be traced over a long interval if the Nicholas Steno basins were open. For instance, Bed F can be traced continuously to the smaller basin in the figure below. The other beds below F can then be correlated to Beds A-E. The Principle of Faunal Succession was later added by William Smith in the late 1700's who observed and studied fossils embedded in rock layers. This principle states that the oldest fossils in a series of sedimentary rock layers will be found in the lowest layer (layer A). Progressively younger fossils occur in higher layers (layer B). This is the same concept as superposition, but it helped geologists realize that you can look at the age of these layers and assign relative dates. This parallels evolution. Younger organisms replace older organisms as the older ones become extinct. Since organisms change through time, it allows correlation of beds far apart. If the layers have similar fossils, one can deduce that William Smith the layers are the same age. Sedimentary layers Math/Science Nucleus © 2001 4 The principles of stratigraphy help to develop a sequence of rock layers. In the figure to the left, the oldest rocks are on the bottom (sandstones). The sandstones represent rocks deposited in a shallow marine environment. The younger rocks reveal an environment change into a tidal area. Through time the tidal area evolves into a lagoon and then a swamp. The sequence provides information on changing environments through time. Then you can determine the sequences in other places and then correlate one rock type with another. For instance, along the beach in Darwin, Australia you can trace rock layers easily. They look like they match! Darwin, Australia Stratigraphy is important to understand events that happened over time and over a large area, However, to interpret these events you require slices of rocks through time commonly referred to as or cores. Ships can core rock layers from the ocean bottom. Cores would be taken at intervals that can help us correlate and Ship taking a core interpret how the rocks were laid down. In the figure of cores, each core represents a slice of the Earth. In “A,” the green shells are the oldest and the blue seastars are the youngest. You can see that as you go from cores A to D the fauna adds snails to the region. A stratigrapher would determine what caused this sequence of events. Stratigraphers also look at the rocks, the fossils, and other evidence to make these conclusions. Cores Math/Science Nucleus © 2001 5 Interpreting how the Earth’s sedimentary layers have formed, is difficult. Cores taken on land and from the ocean are not only expensive to retrieve, but represent a small percentage of the Earth’s surface. Methods using seismic waves developed in the 1960's help to observe the crust’s layers in detail. Seismic stratigraphy is when energy waves are used to bounce off the different layers of the Earth. These layers provide us with data that a seismic stratigrapher can then interpret. For example, in the seismic profile below we show the results of waves bouncing off the different layers and then recorded on the surface of the Earth. These “wavy” images can then be used to reconstruct the area in rock units, as shown in the interpretation of the seismic profile. These advances have allowed geologists to map more area than Seismic profile of an idealized area ever before. Prior to these advances, only outcrops and geologists walking and recording on their maps could be used. Interpretation of the ideal seismic profile Seismic profiles can be used to help calculate and create three- dimensional models of the Earth’s layers that are hidden from view. New advances in other methods such as infrared profiles and other electromagnetic wave technologies, will allow us one day to see what the crust looks like. Stratigraphy has evolved from an observational science into a high tech search for tools to help us explore what our world looks like. All these new techniques that we are developing will help us find resources like oil and metals. This technology can also be used to learn about the Three dimensional seismic model surface of other planets. Math/Science Nucleus © 2001 6 EARTH SCIENCES - STRATIGRAPHY Lesson 2 - Correlation Activity MATERIALS: Objective: Students practice reader correlation of rock units. colored pencils worksheets Teacher note Rock units without fossils are hard to correlate over large areas that are not visually continuous. Visual correlation of rock units are difficult because many outcrops may be covered. Erosion after the rocks were deposited may also prevent correlation. Lithologic units (rocks) without fossils can look the same. Sandstones created on the beach could look similar to sands that were deposited offshore in storm conditions or even sand created in deserts.